Thermistors with negative resistance temperature characteristics (NTC) have been widely used as resistors for temperature compensation and for suppressing an inrush current.
As a ceramic material used for NTC thermistors of this sort, a ceramic composition mainly containing Mn is known.
For example, Patent Document 1 discloses a thermistor composition composed of oxides containing Mn, Ni, and Al, the composition having a Mn content of 20% to 85% by mole, a Ni content of 5% to 70% by mole, and an Al content of 0.1% to 9% by mole, with the sum of these contents being 100% by mole.
Patent Document 2 discloses a thermistor composition containing metal oxides, the composition having a Mn content of 50% to 90% by mole and a Ni content of 10% to 50% by mole in terms of metal, with sum of these contents being 100% by mole, in which 0.01% to 20% by weight of CO3O4, 5% to 20% by weight of CuO, 0.01% to 20% by weight of Fe2O3, and 0.01% to 5.0% by weight of ZrO2 are added to the composition.
Patent Document 3 discloses a thermistor composition containing a Mn oxide, a Ni oxide, an Fe oxide, and a Zr oxide, having “a” percent by mole (wherein 45<a<95) of the Mn oxide in terms of Mn and (100−a) percent by mole of the Ni oxide in terms of Ni as main components, in which when the proportion of the main components is defined as 100% by weight, the proportions the other components are as follows: 0% to 55% by weight of the Fe oxide in terms of Fe2O3 (provided that 0% by weight and 55% by weight are excluded) and 0% to 15% by weight of the Zr oxide in terms of ZrO2 (provided that 0% by weight and 15% by weight are excluded).
Non-Patent Document 1 reports that when Mn3O4 is gradually cooled (at a cooling rate of 6° C./hr) from a high temperature, plate crystals are formed. It also reports that when rapid cooling from a high temperature in air, although the plate crystals are not formed, a lamella structure (streak-like contrast) appears.
Furthermore, Non-Patent Document 1 reports that when Ni0.75Mn2.25O4 is gradually cooled from a high temperature (at a cooling rate of 6° C./hr), a single spinel phase is formed, and plate-like precipitates and a lamella structure are not observed. For rapid cooling from a high temperature in air, although the plate-like precipitates are not formed, the lamella structure appears.
That is, Non-Patent Document 1 describes that for Mn3O4 and Ni0.75Mn2.25O4, a change in the cooling rate from a high temperature results in textures having different crystal structures. In addition, Non-Patent Document 1 describes that for Mn3O4, in order to obtain plate-like precipitates, it is necessary to slow cooling from a high temperature to a cooling rate of about 6° C./hr.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 62-11202    [Patent Document 2] Japanese Patent No. 3430023    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2005-150289    [Non-Patent Document 1] J. J. Couderc, M. Brieu, S. Fritsch and A. Rousset. Domain Microstructure in Hausmannite Mn3O4 and in Nickel Manganite, Third Euro-Ceramics VOL. 1 (1993) p. 763-768.